Vanadium pentoxide-tungsten trioxide catalyst supported on iron ion-exchanged titanium dioxide and method for removing nitrogen oxides using the same

ABSTRACT

The present invention pertains to: a vanadium pentoxide-tungsten trioxide catalyst supported on an iron ion-exchanged titanium dioxide; and a method for removing nitrogen oxides using the same. More specifically, the present invention pertains to: a deNOxing catalyst in which the iron ion-exchanged titanium dioxide is utilized as a support for the vanadium pentoxide and tungsten trioxide to drastically reduce the generation and emission of nitrous oxide; and a method for removing nitrogen oxides using the same.

TECHNICAL FIELD

The present invention relates to a vanadium pentoxide (V₂O₅)-tungsten trioxide (WO₃) catalyst supported on an iron ion-exchanged titanium dioxide and a method for removing nitrogen oxides (NO_(x)=NO+NO₂) using the same and, more specifically, to a deNO_(x)ing catalyst that dramatically reduces the formation and emission of nitrous oxide (N₂O) using the iron ion-exchanged titanium dioxide as a support of vanadium pentoxide and tungsten trioxide and a method for removing the NO_(x).

BACKGROUND ART

Nitrogen oxides, which are mainly emitted from the combustion process of fossil fuels, such as coal, oil, and natural gas, and exist in the atmosphere, are one of the precursors to photochemical smog in metropolises. NO itself has a strong toxic effect on the human respiratory system and produces more toxic photochemical oxidants via photochemical reactions in the atmosphere. Selective catalytic reduction (SCR) that utilizes ammonia (NH₃) or urea as a reducing agent has been most widely adopted to control NO_(x) emissions in large-scale stationary sources, such as thermal power plants, industrial boilers, waste incinerators, etc.

SCR technology capable of effectively removing NO_(x) existing in flue gases with large volumetric flow rates, using NH₃ or urea as a reducing agent (typically called NH₃-SCR or urea-SCR) mostly adopts V₂O₅—WO₃/TiO₂ and V₂O₅—MoO₃/TiO₂ catalysts. In NO_(x)—NH₃—O₂ reaction over the SCR deNO_(x)ing catalysts, a significant amount of N₂O is emitted as a byproduct.

N₂O is one of the six kinds of greenhouse gases (CO₂, CH₄, N₂O, HFCs, PFCs, SF₆) which have been well known to cause global warming. The CO₂ has a global warming potential (GWP) of 1 whereas N₂O has a GWP of 310. Thus, the contribution of N₂O to the extent of the global warming is 310 times that of CO₂, at the same concentration level.

Accordingly, many areas including the United States and the European Union have regulated greenhouse gases emissions including N₂O. South Korea has adopted the greenhouse gas emissions trading scheme to regulate greenhouse gas emissions, such as CO₂ and N₂O.

Many studies have reported a catalyst and method to minimize the formation of N₂O in NH₃-SCR processes. Krocher and Elsener, Ind. Eng. Chem. Res., 47 (2008) 8588, reported a configuration of V₂O₅—WO₃/TiO₂ and Fe-ZSM-5 for lowering N₂O production levels in NH₃-SCR reaction. A series combination of the Fe-ZSM-5 following the former catalyst had a suppressible effect on the N₂O formation; however, low NO_(x) efficiencies were shown irrespective to the packing order, compared to those of the supported V₂O₅-based catalyst. One research group of applicants, Catal. Commun., 86 (2016) 82, proposed Fe-ZSM-5-coated V₂O₅—WO₃/TiO₂ catalysts which could greatly suppress N₂O formation but caused a significant decrease in NO removal activity almost at all reaction temperatures depending on the coating content. According to a study of 1% V₂O₅-10% WO₃ dispersed on Fe₂O₃/TiO₂ with the Fe₂O₃ amounts of 1-5% by Zhang and coworkers, Catal. Sci. Technol., 3 (2013) 191, all these catalysts showed a less N₂O formation at temperatures >450° C., compared to the bare TiO₂-supported catalyst. Recent reports by Kim and coworkers, Catalysts, 8 (2018) 134, Catal. Today, 360 (2021) 305 and 375 (2021) 565, studied effects on N₂O production levels in NH₃-SCR reaction at 200-480° C. over V₂O₅—WO₃/TiO₂ catalysts onto which Fe₂O₃ with different moieties had been added. This attempt could yield a great depression of the N₂O formation at high temperatures >400° C. at which a significant decrease in deNO_(x)ing efficiency was however shown, depending on the iron oxide loading.

EP3689441A1 is directed to a process of reducing NO_(x) in a source gas, comprising passing the gas over a catalyst suitable for selective catalytic reduction of NO_(x) and in the presence of a reducing agent, wherein the catalyst is a ferrierite (FER) zeolite which is not loaded with iron and is not loaded with any transition metal. This process can comprise passing the gas over an N₂O decomposition catalyst in at least one deN₂O stage which is before or after a deNO_(x) stage. The simultaneous removal of N₂O and NO_(x) is performed in at least one catalyst bed containing the catalyst which is not loaded with iron and transition metals.

KR10-1933227 by Shell Internationale Research Maatschappij B.V. describes a method of removing N₂O in an N₂O-containing gas stream. Wherein said method mainly comprises heating the gas stream at a heat exchange region and decomposing the N₂O by contacting the gas stream with an N₂O decomposition catalyst which utilizes zeolites with a noble metal selected from the group of ruthenium, rhodium, silver, rhenium, osmium, iridium, platinum and gold, and with a transition metal selected from the group of vanadium, chromium, manganese, iron, cobalt, nickel and copper.

Disclosed in U.S. Pat. No. 7,485,276B2 and KR10-0723819B1 by ThyssenKrupp Industrial Solutions AG is a method for removing NO_(x) and N₂O from a residual gas stream in nitric acid production process. The method comprises a deNO_(x)ing stage in which the NO_(x) is removed by an NH₃-SCR process and then a secondary deN₂O stage in which a gas stream containing NO_(x) and N₂O with an NO_(x)/N₂O ratio of 0.001-0.5 is contacted with a catalyst at gas temperatures ranging from 350-500° C. to reduce the N₂O with a further removal of NO wherein the catalyst is Fe-zeolites, preferably Fe-ZSM-5.

KR10-1925106 relates to a simultaneous removal of N₂O and NO_(x) in a heat recovery catalytic reactor with a catalyst packed bed. A gas stream containing N₂O and NO_(x) passes through the reactor at 320-450° C. wherein the pollutants are removed by selective catalytic reduction with NH₃ in the catalyst bed which is packed with the prior art catalysts.

Provided in U.S. Pat. No. 10,022,669B2 by ThyssenKrupp Industrial Solutions AG is a process for removing N₂O and NO_(x) from off gases. According to the invention, the process comprises catalytic decomposition of the N₂O by means of iron-containing zeolite catalysts (deNO_(x) stage) and catalytic reduction of the NO_(x) by means of reducing agents (deN₂O stage) which is placed upstream of the deNO_(x) stage. This staged combination can allow the downstream deNO_(x)ing process to be operated under optimal conditions.

Conventional V₂O₅—WO₃/TiO₂ catalysts which have been commercially proven in a variety of deNO_(x) applications can generate significant concentrations of N₂O having the high GWP value in NH₃-SCR deNO_(x)ing processes. Many efforts have been devoted to develop new catalysts for depressing such an N₂O formation and methods of using the same in the related industries and academic institutions to date. Some Fe-ZSM-5- and Fe₂O₃-promoted V₂O₅/TiO₂-based catalysts could appreciably lower the extent of the formation of N₂O in NH₃-SCR reaction but gave an adverse effect on NO_(x) removal activity, as stated previously. Of course, a suitable catalyst for removing N₂O can be installed downstream of deNO_(x)ing processes, similar to a concept of the staged NO_(x)/N₂O removal described in the aforementioned patents. However, this requires additionally a deN₂O catalyst.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

To address the foregoing issues, an objective of the present invention is to provide a vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide, which has been utilized as a support for vanadium pentoxide and tungsten trioxide to dramatically reduce N₂O formation and emissions, and a method for removing nitrogen oxides using the same.

Another objective of this invention is to provide the iron ion-exchanged titanium dioxide-supported vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst whose deNO_(x)ing performance is, even with its primary role for the depression of N₂O formation and emissions, comparable to those of commercial deNO_(x)ing catalysts, and a method for removing NO_(x) using the same.

Technical Solution

In the present invention to achieve the foregoing objectives, a catalyst to selectively remove nitrogen oxides from stationary sources by NH₃ or urea as a reducing agent relates to an iron ion-exchanged titanium dioxide-supported vanadium pentoxide-tungsten trioxide catalyst. Herein, the iron ion-exchanged titanium dioxide support is characterized by modifying and functionalizing the surface of a titanium dioxide, mixing with a precursor of iron ions and then exchanging with the iron ions at a sublimation temperature using an ion exchange technique.

The iron ions are divalent iron ions.

The surface of the titanium dioxide is functionalized with hydroxyl groups.

The ion exchange technique is a solid-state ion exchange.

The iron ions are included at 0.5 to 5 wt % with respect to the titanium dioxide.

The vanadium pentoxide is included at 0.2 to 3 wt % with respect to the titanium dioxide.

The tungsten trioxide is included at 2 to 30 wt % with respect to the titanium dioxide.

The catalyst can greatly lower a level of N₂O produced as a byproduct in the NO_(x) removal reaction.

Further, this invention provides a method for removing NO_(x) using the vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on the iron ions-exchanged titanium dioxide.

Effects of the Invention

As described above, according to the iron ions-exchanged titanium dioxide-supported vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst and the method for removing NO_(x) using the same, it is possible to save operating expenses of deNO_(x)ing facilities since N₂O formation and emissions are dramatically reduced thereby lowering unnecessary consumption of the reducing agents.

According to the vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on the iron ion-exchanged titanium dioxide and the method for removing NO_(x) using the same, although this invention has allowed the catalyst to primarily depress N₂O formation and emissions, it is also possible to provide deNO_(x)ing performance that is comparable to that of commercial deNO_(x)ing catalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing N₂O formation levels of a sample of a vanadium pentoxide-tungsten trioxide catalyst supported on the iron ion-exchanged titanium dioxide according to the present invention.

FIG. 2 is a graph showing deNO_(x)ing performance of a sample of a vanadium pentoxide-tungsten trioxide catalyst supported on the iron ions-exchanged titanium dioxide according to the present invention.

METHOD TO PRACTICE THE INVENTION

Specific features and advantages of the present invention will be described in detail with the drawings, embodiments, and comparative examples below. In the case that detailed descriptions of the functions and features regarding the present invention make the gist of this invention unclear, embodiments will substitute for them.

The present invention relates to a vanadium pentoxide-tungsten trioxide catalyst supported on an iron ion-exchanged titanium dioxide and a method for removing NO_(x) using the same, and more specifically, to a deNO_(x)ing catalyst that dramatically reduces the formation and emission of N₂O using the iron ion-exchanged titanium dioxide as a support of vanadium pentoxide and tungsten trioxide and a method for removing NO_(R).

The deNO_(x)ing catalyst may be a V₂O₅—WO₃/TiO₂ containing tungsten trioxide as a promoter or a V₂O₅/TiO₂ without the promoter. However, considering the durability and activity of the catalyst, it is preferable that tungsten trioxide as a promoter is contained.

The titanium dioxide is not limited, if commonly used in the technical field of NO_(x) removal, to a specific one, and would more preferably be an anatase-type titanium dioxide rather than a rutile-type one. Anatase-type titanium dioxide, whose band gap between the valence and conduction bands is relatively large compared to that of rutile-type titanium dioxide, possesses a high redox potential and is relatively more able to stably maintain the dispersion of the catalyst components.

An iron ion-exchanged titanium dioxide support is prepared by introducing iron ions into a titanium dioxide, whose surface has been modified and functionalized, using an ion exchange method that allows the exchange between iron ions existing in their precursor and hydroxyl groups.

The surface of the titanium dioxide is modified and functionalized with the hydroxyl groups to enhance the ion exchange capacity of iron ions, prior to introducing the iron ions.

As a specific method for introducing hydroxyl groups onto the surface of the titanium dioxide, this is treated in an ammonium hydroxide (NH₄OH) solution with a desired hydrogen ion concentration (pH). However, any method for introducing the hydroxyl groups onto the surface of the titanium dioxide can be utilized without a limitation.

Then, iron ions are exchanged with the hydroxyl groups in the functionalized titanium dioxide thereby significantly reducing the formation of N₂O itself, generating the active sites, on which the removal reaction of the N₂O may occur independent of the deNO_(x)ing reaction, and reducing N₂O via reaction between N₂O formed and NH₃ adsorbed on the catalyst surface in the deNO_(x)ing reaction.

In other words, the method of the present invention is not a method for removing N₂O formed and emitted already as in the prior art N₂O reduction technology. Not only can the method suppress N₂O formation itself on the surface of the deNO_(x)ing catalyst, but it can also decompose N₂O at the active sites of the iron ions-exchanged titanium dioxide or reduce N₂O using excessive NH₃ residues on the catalyst surface before the N₂O is desorbed and emitted when a small amount has been formed on the surface of the catalyst.

The introduction of iron ions onto the titanium dioxide functionalized with hydroxyl groups can utilized the prior art method. As a specific example, one of solid-state ion exchange and wet ion exchange methods can be utilized. Preferably, the solid-state ion exchange method can be applied.

The solid-state ion exchange method mechanically and completely mixes an iron ion precursor with the functionalized titanium dioxide at room temperature, removes oxygen and moisture remaining in the mixture by pretreating it under heated conditions and a flow of an inert gas (N₂, Ar, He, etc.) for a sufficient time, and then allows an exchange of iron ions on the surface of the functionalized titanium dioxide through a solid-state reaction between the iron ions and the hydrogen ions in the hydroxyl groups at temperatures that are equal to or higher than the sublimation temperature of the precursor of the iron ions.

As the precursor of the iron ions, salts in the form of chloride (chlorine salt), nitrate, sulfate, phosphate, carbonate, and derivatives thereof may be used, and preferably, chloride salts that do not cause the deposition of anionic salts on the surface of the titanium dioxide may be primarily used.

In this case, the iron ions may be exchanged to be included in an amount of 0.5 to 5 wt % with respect to the titanium dioxide. In the case that the iron ions are exchanged in an amount less than 0.5 wt % with respect to the titanium dioxide, it is difficult to expect the removal of N₂O and its emission reduction since the surface density of the catalytically active sites in which the N₂O removal reaction may occur. In the case that the iron ions exceed 5 wt % with respect to the titanium dioxide, iron oxides (FeO_(x)) may be formed on the surface of the titanium dioxide and it is difficult to expect a reduction in N₂O emissions. Therefore, it is preferable in the above range.

Precursors of vanadium pentoxide are selected without limitation so long as they are commonly used as a main active component for the removal of NO_(x) in the deNO_(x)ing process.

In this case, the content of the vanadium pentoxide supported on the iron ion-titanium dioxide is not particularly limited so long as it is in the range of the content being normally loaded in the art. The vanadium pentoxide is included at 0.2 to 3 wt % with respect to the iron ion-titanium dioxide. In the case that the content of the vanadium pentoxide is less than 0.2 wt %, sufficient deNO_(x)ing performance cannot be obtained, and in the case of exceeding 3 wt %, the formation of N₂O greatly increases due to NH₃ oxidation in a high temperature region of 350° C. or higher. Therefore, it is preferable in the above range.

Tungsten trioxide is added as a promoter to prevent the phase transition of titanium dioxide and to enhance its surface acidity. Further, the tungsten trioxide plays a role in depressing the oxidation of sulfur dioxide (SO₂) thereby enhancing the durability of the catalyst and not only for enhancement in the dispersion of vanadium pentoxide but also for the suppression of its sintering. The tungsten trioxide may be, to a different content, loaded into the deNO_(x)ing catalyst of the present invention depending on the compositions of flue gases.

In this case, the tungsten trioxide may be included in an amount of 2 to 30 wt % with respect to the iron ion-titanium dioxide. If the tungsten trioxide is added at less than about 2 wt %, it is difficult to expect the aforementioned effects, while in the case of exceeding about 30 wt %, not only the particle size of the tungsten trioxide increases, but the effective supporting of vanadium pentoxide may also be limited. Thereby, the surface structure of tungsten trioxide and vanadium pentoxide promotes side reactions, i.e., the oxidation of NH₃ and of SO₂ into sulfur trioxide (SO₃). Therefore, it is effective in the aforementioned range. Preferably, the tungsten trioxide may be in the range of 3 to 10 wt % with respect to the iron ion-titanium dioxide.

The method for supporting vanadium pentoxide and tungsten trioxide on the iron ion-titanium dioxide can utilize techniques being commonly used in the art and is not particularly limited, e.g., the so-called wet impregnation technique may be used. As a specific example, a 5 wt % oxalic acid (H₂C₂O₄) solution is prepared, and ammonium metatungstate ((NH₄)₆H₂W₁₂O₄₀), which is the precursor of tungsten trioxide, is put into the solution and completely dissolved. An iron ion-titanium dioxide support is put into the prepared precursor solution, the metatungstate is impregnated on the support under stirring in a vacuum rotary evaporator, and tungsten trioxide on the surface of the iron ion-titanium dioxide support (tungsten trioxide/iron-titanium dioxide) is formed by drying, and calcination at 400 to 600° C. After this, an aqueous solution of ammonium metavanadate (NH₄VO₃) is prepared, similar to the preparation of the ammonium metatungstate solution. The tungsten trioxide/iron ion-titanium dioxide catalyst is put into that solution and mixed thoroughly, followed by drying and calcination in a manner similar to that described above, thereby resulting in a vanadium pentoxide-tungsten trioxide/iron ion-titanium dioxide catalyst.

In this case, as described above, the content of the tungsten trioxide loaded into the deNO_(x)ing catalyst of the present invention can be varied. Unlike the aforementioned impregnation order, the support can be sequentially impregnated with precursor solutions of vanadium pentoxide and tungsten trioxide or it is safe to simultaneously impregnate the support with each solution dissolving the two precursors separately. Alternatively, it is possible for the impregnation to be simultaneously conducted by putting and dissolving the two precursors in an oxalic acid solution at once.

A method for removing NO_(x) according to the present invention is described below.

The NO_(x) removal method according to the present invention utilizes a vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on the iron ion-exchanged titanium dioxide. When applying the deNO_(x)ing catalyst according to the present invention to the NO_(x) removal reaction, the reaction temperature is in the range of 150 to 700° C., preferably 150 to 600° C., and within this temperature range, it is possible to achieve superior NO_(x) removal activity and prevent excess formation of N₂O.

Gas hourly space velocities (GHSV) are not particularly limited, however, these are preferable 2000 to 500,000 h⁻¹ based on the catalyst powder, more preferably 10,000 to 300,000 h⁻¹. If the space velocity is less than 10,000 h⁻¹, catalytic activity is excellent but catalyst costs become excessively large since necessary volumes of the catalyst increase. If the space velocity exceeds 300,000 h⁻¹, it is difficult to expect deNO_(x)ing performances to the level required in the industrial field.

Oxygen concentrations in reactant gases for the catalytic reaction may be maintained at 0.1 to 21 vol %, preferably 1 to 15 vol %, but that is not particularly limited.

Hereinafter, the present invention is described in detail with references and embodiments thereof. However, the following embodiments are to specifically illustrate the present invention rather than being limited thereto.

1. Preparation of an Iron Ion-Exchanged Titanium Dioxide Support

A method for preparing an iron ion-exchanged titanium dioxide (iron ion-titanium dioxide) for supporting vanadium pentoxide and tungsten trioxide to dramatically reduce N₂O formation levels of a vanadium pentoxide/titanium dioxide-based catalyst in an NH₃-SCR deNO_(x)ing reaction in the present invention is as follows.

150 g of titanium dioxide dried at 110° C. for 8 hours was slowly put into a solution which had been adjusted to a pH=11 by sparingly adding an ammonium hydroxide solution to 100 mL of distilled deionized water in a flat-bottom flask under stirring at 300 rpm, and was stirred at 600 rpm. After stirring for 2 hours, the pH of the mixture was measured using a pH meter, the ammonium hydroxide solution was again added sparingly so as to have the pH=11, and the mixture was continuously stirred for 8 hours and filtered through a filter paper.

The cake filtered on the filter paper was again put in a beaker, 1 L of distilled deionized water was added thereto, and after stirring at 600 rpm for 1 hour, this mixture was filtered again. The sample was washed by repeating the above filtration process until the pH of the filtrate is the same as that of the distilled water. The resulting cake was dried in a drying oven at 110° C. for 12 hours and was powdered, and was designated as “H—TiO₂.”

10 g of the H—TiO₂ and an amount of ferrous chloride (FeCl₂) corresponding to a desired content of iron ions were put in a vial and mixed using a ball mill for 2 hours. This mixture was put in a quartz reactor, treated in a flow of 100 cm³/min of nitrogen for 8 hours, heated to a desired sublimation temperature (200 to 550° C.) at a temperature ramping rate of 3° C./min and then maintained at the chosen temperature for 5 hours to perform solid-state ion exchange. After this process was completed, the sample was recovered and made into a powder again. The iron ion-exchanged titanium dioxide supports were designated as x % Fe—TiO₂-n in the following embodiments, where x denotes the wt % of the iron ions, and n denotes the sublimation temperature at which the solid-state ion exchange was conducted.

2. Preparation of Vanadium Pentoxide-Tungsten Trioxide/Iron Ion-Titanium Dioxide deNO_(x)ing Catalysts

The method for preparing supported tungsten trioxide and vanadium pentoxide catalysts in the present invention is as follows. As a representative example, a method for impregnating a 3.15% Fe—TiO₂-200 support with the active substances is described in detail.

10 g of 3.15% Fe—TiO₂-200 is put in a vial and then dried in a drying oven at 110° C. for about 8 hours. 14 mL of distilled water was put in a beaker, an amount of ammonium metatungstate corresponding to a desired content of tungsten trioxide was accurately measured and completely dissolved, and the dried 3.15% Fe—TiO₂-200 and the precursor solution were put in a round flask and well mixed at 120 rpm using a vacuum rotary evaporator for 1 hour. After this, a residual solution was completely evaporated by operating a vacuum pump equipped with an evaporator, and the sample was recovered, dried in a drying oven at 110° C. for 8 hours, and then calcined at 500° C. for 1 hour in a quartz reactor.

5 g was sampled from a vial in which the tungsten trioxide-impregnated iron ion-titanium dioxide catalyst (7.8% WO₃/3.15% Fe—TiO₂-200) prepared as described above was stored. After a 5-wt % oxalic acid solution was made by dissolving oxalic acid in 14 mL of distilled water, an amount of ammonium metavanadate corresponding to 1.6 wt % vanadium pentoxide was put into the solution and completely dissolved. Both the aforementioned catalyst and the vanadium precursor solution were put in a round flask and mixed well for 1 hour at 120 rpm by using a vacuum rotary evaporator. After evaporating a residual solution, it was put in a drying oven, dried at 110° C. for 8 hours and calcined at 500° C. for 1 hour, thereby obtaining a 1.6% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 catalyst.

Catalysts having different contents of iron ions, tungsten trioxide, and vanadium pentoxide used in the embodiments of the present invention were prepared in the same manner as that described above.

3. Evaluations of N₂O Reduction and NO_(x) Removal 0.4 g of each catalyst was loaded into a quartz reactor and calcined again at 500° C. for 1 hour in a flow of a flowing mixture of 21 vol % oxygen and 79 vol % nitrogen at 1 L/min. After lowering the temperature of the catalyst bed to 200° C. and reducing the oxygen concentration to 5 vol %, 500 ppm nitrogen monoxide (NO) and 500 ppm NH₃ were added to the reaction gas.

The gas flow corresponding to a space velocity of 76,200 h⁻¹ based upon the amount of the catalyst used and the total flow rate was passed through the catalyst bed while allowing them to react at desired reaction temperatures (200 to 480° C.). After the reaction at each reaction temperature, concentrations of N₂O formed and unreacted NO and NH₃ were measured by Fourier transform infrared spectroscopy (FT-IR). The gas flowing tube and gas cell were heated to 175° C. to prevent the gas-phase reactions between the reactants and the condensation of moisture generated in the catalytic reaction, and 15 L/min of dry air from which moisture had been completely removed was continuously flowed into the interior of the FT-IR to exclude the influence of moisture. The purities of NO and NH₃ used in the present invention were 99.99 vol % and 99.999 vol %, respectively. Concentrations of N₂O and NO were calculated by using a calibration curve expressed as each gas-phase concentration vs. the area of a corresponding characteristic peak in the spectrum obtained by FT-IR, and the NO removal efficiency was estimated as the ratio of the concentration of NO remaining after the reaction to the initial concentration.

In summary, the evaluations of N₂O reduction performance and deNO_(x)ing efficiency of catalysts and reference ones belonging to the present invention were conducted under NH₃-SCR reaction conditions including 5 vol % oxygen, 500 ppm NH₃, 500 ppm NO, and a space velocity of 76,200 h⁻¹.

FIG. 1 and FIG. 2 show the N₂O formation level and deNO_(x)ing efficiency of 1.6 wt % vanadium pentoxide-7.8 wt % tungsten trioxide catalyst (1.6% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200) supported on the iron ion-exchanged titanium dioxide according to the present invention (Embodiment 1 of Table 1).

Embodiment 1

In Table 1, N₂O formation levels and deNO_(x)ing performance over a 1.6% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 catalyst were compiled as a function of reaction temperature. Even at a reaction temperature of 480° C., the concentration of N₂O formation over the catalyst was only 5 ppm. The deNO_(x)ing performance of the catalyst was found to be 85% or more at a reaction temperature of 250 to 450° C.

TABLE 1 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Reaction temperature (° C.) Reaction temperature (° C.) Embodiment Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 1 1.6% 36 85 97 99 100 89 75 0 0 0 0 2 4 5 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200

In FIGS. 1 and 2 , as a comparison group to compare the formation levels of nitrous oxide in regard to whether or not the introduction of iron ions into a titanium dioxide, N₂O formation levels, and deNO_(x)ing efficiency of a catalyst where vanadium pentoxide and tungsten trioxide had been, in the same amount as that described above, supported on the titanium dioxide without iron ion exchange (1.6% V₂O₅-7.8% WO₃/TiO₂) were also shown (Comparative Example 1 in Table 2).

Comparative Example 1

Table 2 shows N₂O formation levels and deNO_(x)ing performance of the 1.6% V₂O₅-7.8% WO₃/TiO₂ catalyst selected as a reference group of the present invention. This catalyst showed the formation of N₂O by a side reaction from 350° C. At 480° C., the amount formed was 110 ppm, which was much higher than that in Embodiment 1. At all temperatures, the deNO_(x)ing efficiency of the aforementioned catalyst was similar to the deNO_(x)ing performance of the catalyst in Embodiment 1.

TABLE 2 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Comparative Reaction temperature (° C.) Reaction temperature (° C.) Example Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 1 1.6% 38 83 100 100 100 92 81 0 0 0 5 15 60 110 V₂O₅-7.8% WO₃/TiO₂

As illustrated in FIGS. 1 and 2 and compiled in Embodiment 1 and Comparative Example 1, when the iron ion-exchanged titanium dioxide support was applied to prepare the deNO_(x)ing catalyst, this maintained deNO_(x)ing performances comparable to those of the commercial deNO_(x)ing catalyst while it could dramatically reduce production levels of N₂O from side reactions.

Provided below are embodiments and comparative examples showing implementations of the present invention.

Comparative Examples 2 to 4

Comparative Examples 2 to 4 in Table 3 show N₂O formation levels and deNO_(x)ing performances of vanadium pentoxide/titanium dioxide-based catalysts chosen as the other references in the present invention. The 1.6% V₂O₅-7.8% WO₃/H—TiO₂ catalyst (Comparative Example 2) exhibited an N₂O formation level similar to that of 1.6% V₂O₅-7.8% WO₃/TiO₂ in high-temperature region, and 1.6% V₂O₅-7.8% WO₃/10% SiO₂—TiO₂ gave an N₂O formation exceeding 150 ppm at 480° C. (Comparative Example 4). The N₂O formation levels of the 1.6% V₂O₅-7.8% WO₃ catalyst supported on 3.16% Fe₂O₃/TiO₂ (Comparative Example 3) were much higher than that in the Embodiment 1.

TABLE 3 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Comparative Reaction temperature (° C.) Reaction temperature (° C.) eExample Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 2 1.6% 40 83 95 100 100 96 90 1 5 2 8 16 62 108 V₂O₅-7.8% WO₃/H—TiO₂ 3 1.6% 37 78 97 100 96 75 58 0 1 2 3 6 21 40 V₂O₅-7.8% WO₃/3.16% Fe₂O₃/TiO₂ 4 1.6% 25 50 70 80 95 70 62 0 0 0 8 20 65 160 V₂O₅-7.8% WO₃/10% SiO₂—TiO₂

As seen in the Embodiment 1 and Comparative Examples 1 to 4, it is seen that the formation of N₂O in NH₃-SCR reaction can be dramatically reduced by exchanging iron ions in the titanium dioxide prior to loading vanadium pentoxide and tungsten trioxide with desired amounts onto it.

Embodiments 2 to 5

As shown in Embodiments 2 to 5 in Table 4, it represented that the N₂O formation levels and deNO_(x)ing performances of the vanadium pentoxide-tungsten trioxide catalyst supported on the iron ion-exchanged titanium dioxide are influenced on the ion exchange temperature (sublimation temperature). In amounts of the vanadium pentoxide, tungsten trioxide, and iron ions that are all the same, the N₂O formation levels and deNO_(x)ing performances in the NH₃-SCR reaction were similar when the ion exchange temperature had been 200 to 550° C. (Embodiments 2 to 4).

TABLE 4 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Reaction temperature (° C.) Reaction temperature (° C.) Embodiment Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 2 1.6% 36 85 92 97 100 87 73 0 0 0 0 2 5 6 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-300 3 1.6% 36 85 93 95 100 86 72 0 0 0 0 3 6 7 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-400 4 1.6% 34 85 92 95 100 87 72 0 0 0 0 3 5 8 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-550 5 1.6% 25 75 87 93 100 75 58 0 0 0 2 7 17 30 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-600

1.6% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-600 (Embodiment 5), which is a catalyst with the titanium dioxide exchanged by iron ions at 600° C., exhibited N₂O formation levels of less than those of 1.6% V₂O₅-7.8% WO₃/TiO₂ (Comparative Example 1) but had larger N₂O formations as well as lower deNO_(x)ing performances, compared to the catalysts ion-exchanged at lower sublimation temperatures. Thus, that temperature is inappropriate as a sublimation temperature for solid-state ion exchange.

Embodiments 6 to 10

According to Embodiment 1 in Table 1 and Embodiments 2 to 5 in Table 4, it is known that the N₂O formation levels and deNO_(x)ing performances in the NH₃-SCR reaction are similar when the sublimation temperature for iron ions exchange was 200 to 550° C. Accordingly, the N₂O formation levels and deNO_(x)ing performances of 1.6% V₂O₅-7.8% WO₃ catalysts supported on titanium dioxides ion-exchanged by iron ions in different contents at a sublimation temperature of 200° C. were examined. Their measuring methods were the same as described in detail above.

As it can be seen by Embodiments 6 to 10 in Table 5, 1.6% V₂O₅-7.8% WO₃/0.2% Fe—TiO₂-200 (Embodiment 6) having an iron ion content of 0.2 wt % had deNO_(x)ing performances similar to those of 1.6% V₂O₅-7.8% WO₃/TiO₂ (Comparative Example 1) but still exhibited a high N₂O concentration, such as 47 to 85 ppm at 450 to 480° C.

TABLE 5 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Reaction temperature (° C.) Reaction temperature (° C.) Embodiment Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 6 1.6% 37 83 98 99 100 93 80 0 0 0 3 10 47 85 V₂O₅-7.8% WO₃/0.2% Fe—TiO₂-200 7 1.6% 35 82 95 99 100 91 78 0 0 0 1 4 7 9 V₂O₅-7.8% WO₃/0.5% Fe—TiO₂-200 8 1.6% 34 84 96 99 100 90 77 0 0 0 0 3 6 8 V₂O₅-7.8% WO₃/1.74% Fe—TiO₂-200 9 1.6% 37 86 97 98 100 87 73 0 0 0 0 3 5 6 V₂O₅-7.8% WO₃/5.02% Fe—TiO₂-200 10 1.6% 35 81 95 98 100 70 50 0 0 0 0 2 5 7 V₂O₅-7.8% WO₃/7.04% Fe—TiO₂-200

It can be seen that the concentration of N₂O formed has been remarkably reduced when the iron ion content is 0.5 to 7.04 wt % (Embodiments 7 to 10). However, as shown from the deNO_(x)ing performances of 1.6% V₂O₅-7.8% WO₃/7.04% Fe—TiO₂-200, if the content of ion-exchanged iron ions is too high, the deNO_(x)ing performances at high temperatures may be rather reduced, and thus it is not preferable (Embodiment 10).

Embodiments 11 to 14

Catalysts having different contents of vanadium pentoxide (0.2 to 5 wt % V₂O₅) were, using 7.8 wt % WO₃ supported on 3.15% Fe—TiO₂-200, prepared according to the same method as the preparation technique applied to the aforementioned embodiments. The N₂O formation concentrations and deNO_(x)ing performances of the catalysts were evaluated by the same method as that provided in Embodiment 1.

As it can be shown by Embodiments 11 to 14 in Table 6, the N₂O formation levels and deNO_(x)ing performances increased with an increase in vanadium pentoxide. Over the 4.3% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 catalyst having a high vanadium pentoxide content, low-temperature deNO_(x)ing performances were excellent but the concentration of N₂O being formed at 400° C. or higher remarkably increased (Embodiment 14).

TABLE 6 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Reaction temperature (° C.) Reaction temperature (° C.) Embodiments Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 11 0.1% 25 55 77 89 95 70 58 0 0 0 0 1 3 4 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 12 0.2% 34 82 94 98 100 85 71 0 0 0 0 2 4 5 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 13 3% 56 92 99 100 100 92 78 0 0 2 4 5 6 9 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 14 4.3% 63 95 99 100 100 95 73 0 2 4 7 22 50 95 V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200

It can be seen that the 0.1% V₂O₅-7.8% WO₃/3.15% Fe—TiO₂-200 catalyst exhibited the lowest N₂O formation level but was not suitable for an NH₃-SCR deNO_(x)ing catalyst because of its relatively low deNO_(x)ing performance compared to that of the other catalysts (Embodiment 12).

Embodiments 15 to 20

Catalysts containing 2 to 29.8 wt % tungsten trioxide were prepared by using 3.15% Fe—TiO₂-200 as a support and 1.6 wt % vanadium pentoxide, and their N₂O formation levels and deNO_(x)ing performances in the NH₃-SCR deNO_(x)ing reaction were investigated as described above.

As shown in Table 7, when an amount of tungsten trioxide was 3 to 10.3 wt % (Embodiments 16 and 17), there was no significant difference not only in N₂O concentrations but also in deNO_(x)ing performances (see Embodiment 1 in Table 1), and a maximum formation amount of N₂O even at 480° C. was just 7 ppm.

TABLE 7 Nitrous oxide concentration DeNO_(x)ing efficiency (%) (ppm) Reaction temperature (° C.) Reaction temperature (° C.) Embodiment Catalyst 200 250 300 350 400 450 480 200 250 300 350 400 450 480 15 1.6% 26 43 62 78 83 74 69 0 0 0 0 3 3 6 V₂O₅-2% WO₃/3.15% Fe—TiO₂-200 16 1.6% 34 82 90 95 100 85 70 0 0 0 0 3 4 7 V₂O₅-3% WO₃/3.15% Fe—TiO₂-200 17 1.6% 37 88 98 100 100 91 77 0 0 0 1 3 5 6 V₂O₅-W.3% WO₃/3.15% Fe—TiO₂-200 18 1.6% 38 89 99 100 100 92 83 0 0 1 9 14 8 6 V₂O₅-15.1% WO₃/3.15% Fe—TiO₂-200 19 1.6% 38 90 99 100 100 95 88 0 0 2 12 25 9 7 V₂O₅-2I.3% WO₃/3.15% Fe—TiO₂-200 20 1.6% 38 92 100 100 100 99 90 0 0 2 15 31 9 7 V₂O₅-29.8% WO₃/3.15% Fe—TiO₂-200

When an amount of tungsten trioxide was 15.1 to 29.8 wt % (Embodiments 18 to 20), high-temperature deNO_(x)ing performances increased compared to those of 1.6% V₂O₅-7.8% WO₃/TiO₂ (Comparative Example 1) but N₂O formation levels at 350 to 400° C. increased remarkably.

Since 1.6% V₂O₅-2% WO₃/3.15% Fe—TiO₂-200 has relatively low deNO_(x)ing performance in the whole reaction temperature range, it can be seen that it is not preferable as a deNO_(x)ing catalyst in spite of showing low N₂O formation concentrations (Embodiment 15).

Although the preferable embodiments and non-preferable ones for implementing the present invention have been compiled above, not only is the present invention not limited thereto, but various modifications may also be made thereto unless they depart from the gist of the present invention.

Various changes and modifications may be made thereto by one of ordinary skill in the art, unless they depart from the technical spirit and scope of the claims. Accordingly, the scope of the present invention should be interpreted by the following claims which have been described to include such changes and modifications. 

1. A vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide for selectively removing nitrogen oxides from stationary sources using ammonia or urea as a reducing agent, wherein the iron ion-exchanged titanium dioxide support is prepared by modifying and functionalizing the surface of a bare titanium dioxide, mixing with a precursor of iron ions, and then exchanging with the iron ions at a sublimation temperature of the precursor using an ion-exchange technique.
 2. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the iron ions are divalent iron ions.
 3. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the surface of the titanium dioxide is functionalized with hydroxyl groups.
 4. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the ion-exchange technique is solid-state ion exchange.
 5. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the iron ions are included at 0.5 to 5 wt % with respect to the titanium dioxide.
 6. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 2, wherein the iron ions are included at 0.5 to 5 wt % with respect to the titanium dioxide.
 7. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the vanadium pentoxide is included at 0.2 to 3 wt % with respect to the titanium dioxide.
 8. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the tungsten trioxide is included at 2 to 30 wt % with respect to the titanium dioxide.
 9. The vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim 1, wherein the supported vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst is able to reduce production levels of nitrous oxide as a byproduct in a removal reaction of nitrogen oxides.
 10. A method for removing the nitrogen oxides using the vanadium pentoxide-tungsten trioxide deNO_(x)ing catalyst supported on an iron ion-exchanged titanium dioxide of claim
 1. 